1. Field of the Invention
The present invention relates to a surface inspection apparatus and a surface inspection method, particularly to a surface inspection apparatus and a surface inspection method for measuring surface conditions of automobile parts, OA apparatus, household electric appliances or the like.
2. Discussion of the Background
In fields of trim parts of automobile, OA, domestic electrification or the like, the level of requirement for physical properties of the surfaces of products concerning their external appearances, in concrete terms, injuries, unevenness in height, unevenness in gloss, unevenness in color, and so forth, has become severer year by year. In particular, in products made of synthetic resins, in addition to the above-described various properties, the external appearance of a weld line, the external appearance of flow marks, stress whitening, and so forth, are greatly concerned in their commodity values.
The surface condition of such a product is hitherto appraised by performing a sensuous test, in which a classification is made on eye observation. In the sensuous test, however, the result of the test is obtained only in a rank on each appraisal item, and minute information on the surface condition, that is, information at the same level as a state of seeing with the naked eye can not be preserved, so it is a hindrance to quality control and material development.
In order to solve such a problem, it has been done to keep the surface condition of a product with a photograph or the like, but it is not considered objective data because its contrast varies according to photographing conditions, printing conditions, and so forth, so a method and an apparatus in which the surface condition of a product or a material can be accurately evaluated or measured have been desired earnestly.
On the other hand, as a method for measuring the surface condition of a synthetic resin material, the present applicant proposed a method for measuring the degree of whitening by an injury in the surface of the material (Japanese Patent Publication No. 52160/1995). This method is a method in which an injury of a predetermined shape is inflicted on a sample made of a synthetic resin, the injured whitening portion is irradiated with a light in dark-field illumination, and the light quantity of the component parallel with the optical axis of the objective lens in diffusedly reflected rays of the light is measured to measure the injury whitening degree. By changing the magnification of the objective lens according to the size of the injury, the light detection extent, that is, the extent in the surface of the sample subject to the measurement of the reflected light is controlled.
This method is useful for a material test in material development or the like, but has a problem that it can not be applied to surface inspection of products in quality control. That is, in this method, because an injury of a predetermined shape must be inflicted on a sample for measurement, the surface condition of the product can not be evaluated as it is, and further, because only the portion of the injury of the predetermined shape is measured due to the low detection accuracy, the surface condition of the whole of the object to be measured including the portion other than the injury can not be measured.
Besides, because an objective lens of a high magnification is used to decrease the detection extent when the whitening degree of a minute injury is measured, in case of a sample having a complicated shape, or the like, if the injury varies in position, a measurement error arises due to a divergence of the focus so a highly accurate measurement can not be realized. If an objective lens of a low magnification is used in order to decrease such a divergence of the focus, the peripheral portion of the injury is included in the detection extent, and so there is a problem that the reflected light from the injury can not be detected with a good accuracy.
It is an object of the present invention to provide a surface inspection apparatus and a surface inspection method in which the surface condition can be measured with a good accuracy independently of the shape of an object to be measured, and which can be applied also to surface inspection of products.
A surface inspection apparatus of the present invention is characterized by comprising a light source for applying a light to a surface of an object to be measured, an objective lens opposite to the surface of the object to be measured and for receiving a reflected light applied from the light source and reflected on the surface of the object to be measured, light detection means for detecting a component incident on the corresponding objective lens from a parallel direction with its optical axis in the reflected light passing through this objective lens and obtaining its light quantity, and a slit provided in the optical path between the objective lens and light detection means.
In the present invention, the light applied from the light source is reflected on the surface of the object to be measured, and the reflected light passes through the objective lens. When the reflected light is incident on the objective lens, the reflected light incident in parallel with the optical axis of the objective lens is incident on the slit after passing through the objective lens, and only the light having passed through the opening of this slit is introduced to the light detection means to obtain its light quantity.
In this manner, because the slit is provided in the light path between the objective lens and the light detection means, since only the component having passed through the opening of the slit in the reflected light reflected in the parallel direction with the optical axis of the objective lens can be taken out and detected, the detection extent of the reflected light in the surface of the object to be measured, namely, the light detection extent can be restricted by the opening of the slit. Accordingly, because the reflected light within the extent limited by narrowing the light detection extent with the objective lens and the slit can be taken out and its light quantity can be obtained, the surface condition of the aimed external appearance can be measured exactly and with a high accuracy.
Further, when the surface condition of the object to be measured of a complicated shape is measured, in case of using the objective lens of a low magnification for preventing a divergence of the focus, because the detection extent in the surface of the object to be measured can be restricted by the slit, since the surface of the object to be measured can be finely divided into a plurality of light detection extents and detected, the surface condition of the object to be measured can be measured with a good accuracy.
Because a superior detection accuracy can be obtained in this manner, not only the conventional degree of whitening of an injury but also a difference in the surface condition due to color, unevenness in height, or the like can be measured, and because the whole of the surface of the object to be measured including an uninjured portion can be measured, in addition to that an evaluation result corresponding to the external appearance of the object to be measured can be obtained, since it is avoided to inflict a predetermined injury on the surface of the object to be measured for measurement as a conventional manner, it can be applied to not only a surface inspection of a material but also a surface inspection in quality inspection of products.
Besides, when the object to be measured is evaluated with scanning, it is desirable to narrow the measurement area, that is, the light detection extent. At this time, because there is a fear of movement of focus with scanning, it is preferable for suppressing the movement of focus to narrow the light detection extent by using the objective lens of a magnification as low as possible and decreasing the opening of the slit.
Besides, because the objective lens of a lower magnification gets nearer to the external appearance of eye observation, a high correlation with the eye observation result can be obtained. Accordingly, it is desirable to use the objective lens of a low magnification, for example, not more than 10, particularly, 1 to 5.
Here, when the magnification is 1, a plane glass board merely transmitting light, an optical fiber cable or the like may be used as the objective lens, or the objective lens itself may not be used.
That is, it is a surface inspection apparatus characterized by comprising a light source for applying a light to a surface of an object to be measured, a tubular member, for example, an optical fiber cable opposite to the surface of said object to be measured and for receiving a reflected light applied from said light source and reflected on the surface of said object to be measured, light detection means for detecting a component incident on this tubular member from a specified direction in the reflected light and obtaining its light quantity, and a slit provided in the optical path between said tubular member and light detection means.
Further, in the apparatus of the present invention, if the spectral characteristic of the light source and the spectral sensitivity of the light detection means are provided into standard ones by selecting the light source and the light detection means, the light quantity can be obtained as a standardized value such as color-difference and lightness.
In the above, it is desirable that illumination switchover means is provided in the light path between the light source and object to be measured, and this illumination switchover means is to switch over bright-field illumination in which the light from the light source is made parallel with the optical axis of the objective lens and applied to the object to be measured through the objective lens, and dark-field illumination in which the light from the light source is made ringlike and applied obliquely with respect to the optical axis of the objective lens such that there is a focus on the surface of the object to be measured.
In this case, with the illumination switchover means, by properly using bright-field illumination and dark-field illumination according to the surface condition of the object to be measured or an aimed item to the surface condition, the surface condition of the object to be measured can be measured with a higher accuracy.
For example, in case of the object to be measured being made of a synthetic resin, when it is inspected on the presence of an injury, the size of the injury or the like, because the light becomes easier to be diffusedly reflected on the injury or the like by using dark-field illumination, in which the light is applied from an oblique direction, the detected light quantity increases and decreases greatly according to the injury, and a superior correlation with the eye observation result can be obtained. On the other hand, when it is inspected on unevenness in lightness, unevenness in gloss, the external appearance of a weld, the external appearance of flow marks, or the like, if bright-field illumination and dark-field illumination are selected according to the eye observation and used, a highly accurate inspection result corresponding to the eye observation can be obtained on each item.
Besides, in dark-field illumination, because the surface of the object to be measured is irradiated with the light in a ringlike manner from all directions, a measurement error due to difference in irradiation direction can be dissolved and a superior detection result correlated with the eye observation result can be obtained. That is, if the light is applied to the object to be measured only from one direction, because inferiority in the external appearance or the like has directivity in general, there is a case that the detected light quantity of the reflected light is different from the case of applying the light from the other direction, besides, because the direction of viewing the object to be measured in the eye observation judgement is not always fixed, by applying the light in the ringlike manner, scatter in data according to irradiation directions can be dissolved.
Further, when an abnormal portion in the surface of the object to be measured is measured by using dark-field illumination, it is desirable to set up the irradiation angle with the light with respect to the optical axis of the objective lens on the basis of the eye observation result. In concrete words, it is desirable that the abnormal portion of the object to be measured is observed with the naked eye with varying the angle, an angle condition in which a difference from the normal portion can be notably distinguished is selected, and this selected angle is used as the irradiation angle with the light. Thereby, because the surface condition can be observed under the same condition as the eye observation, a correlation with the eye observation result can be obtained.
It is desirable that the size of the opening of the above-mentioned slit is changeable.
Thereby, because the light detection extent in the surface of the object to be measured can be voluntarily controlled according to the surface condition of the object to be measured or the like by controlling the size of the opening of the slit, a more highly accurate inspection can be realized.
As the slit, for example, one having a circular opening can be used, and in this case, the diameter of the opening of the slit is desirably 0.2 to 30 mm. If the diameter of the opening of the slit is less than 0.2 mm, the light introduced into the light detection part is insufficient and there arises a fear that it becomes hard to measure.
Besides, when one on the surface of which uneven crimps are given is used as the object to be measured, it is preferable to broaden the light detection extent by increasing the diameter of the opening of the slit because a variation in measured value decreases.
Further, it is desirable that the above-mentioned light detection means comprises calculation means for converting the light quantity of the light having passed through the slit on the basis of a light quantity detected when a standard sample is used as the object to be measured.
Here, the standard sample is the sample used as a standard respectively in properties liked to inspect on the object to be measured, in concrete words, lightness, degree of white, color difference, or the like, for example, a standard sample that is made in common on the basis of a standard color chip of color such as white standard board and Munsell chroma, a standard sample set up individually, the normal portion in the object to be measured, or the like. This standard sample may be properly selected according to the evaluation item.
If such calculation means is provided, the light quantity detected on the object to be measured can be obtained as a relative value based on the light quantity of the standard sample. That is, not only an absolute measurement but also a comparative measurement can be performed. Accordingly, the inspection result can be obtained with a good reproducibility as a value near to the eye observation and easy to understand.
Besides, if one of a good eye observation result is used as the standard sample, because a relative inspection result based on this standard sample is obtained, the object to be measured can be evaluated with ease.
Further, when the inspections are performed with varying the measurement conditions, by using the same standard sample under each measurement condition and converting the light quantity of the object to be measured on the basis of the light quantity of this standard sample, because an error in the inspection result due to the difference in the measurement condition can be dissolved, an accurate inspection result is obtained.
On the other hand, a surface inspection method of the present invention is characterized in that a surface of an object to be measured is irradiated with a light and the irradiation light is reflected on the surface of the object to be measured, in this reflected light, a component parallel with the optical axis of an objective lens provided oppositely to the object to be measured is made incident on a slit through the objective lens, in this incident light, only a component having passed through an opening of said slit is received, and the light quantity of this received light is obtained.
In the present invention, since only the component having passed through the opening of the slit in the reflected light reflected in the parallel direction with the optical axis of the objective lens can be taken out and detected, the detection extent of the reflected light in the surface of the object to be measured, namely, the light detection extent can be restricted by the opening of the slit. Accordingly, because the reflected light within the extent limited by narrowing the light detection extent with the objective lens and the slit can be taken out and its light quantity can be obtained, the surface condition can be measured exactly and with a high accuracy.
Besides, even in case of using the objective lens of a low magnification, because the detection extent in the surface of the object to be measured can be restricted by the slit, the surface condition of the object to be measured can be measured with a good accuracy.
Further, because a superior detection accuracy can be obtained, not only the conventional degree of whitening of an injury but also even a shining injury (injury conspicuous due to increase in gloss of the injured portion, particularly, injury conspicuous upon holding the object to be measured to the light) can be measured. Besides, also a difference in the surface condition due to color, unevenness in height, or the like can be measured, and because the whole of the surface of the object to be measured including an uninjured portion can be measured, an evaluation result corresponding to the external appearance of the object to be measured can be obtained.
Further, since it is avoided to inflict a predetermined injury on the surface of the object to be measured for measurement as a conventional manner, it can be applied to not only a surface inspection of a material but also a surface inspection in quality inspection of products.
It is desirable that the light detection extent in the surface of the object to be measured is controlled by changing the size of the opening of the above-mentioned slit and the magnification of the objective lens, respectively.
Because the light detection extent in the surface of the object to be measured can be voluntarily controlled according to the surface condition of the object to be measured or the like by combining the size of the opening of the slit and the magnification of the objective lens in this manner, a more highly accurate inspection can be realized.
Further, it is desirable that the light quantity of the received light is converted on the basis of a light quantity detected when a standard sample is used as the object to be measured.
Thereby, because the light quantity of the light received on the object to be measured can be obtained as a relative value based on the standard sample, the reliability in the value of each measurement is high and the inspection result can be obtained with a good reproducibility as a value near to the eye observation and easy to understand.
Besides, if one of a good eye observation result is used as the standard sample, because a relative inspection result based on this standard sample is obtained, the object to be measured can be evaluated with ease.
Further, when the inspections are performed with varying the measurement conditions, by using the same standard sample under each measurement condition and converting the light quantity of the object to be measured on the basis of the light quantity of this standard sample, because an error in the inspection result due to the difference in the measurement condition can be dissolved, an accurate inspection result is obtained.
The irradiation angle with the light to the object to be measured may be changed according to the surface condition of the object to be measured.
There is no particular limit in this irradiation angle, and it may be voluntarily set up according to the object of the inspection and the surface condition of the object to be measured within a range of 0 to 45 degrees with respect to the optical axis of the objective lens.
By controlling the irradiation angle with the light in this manner, the surface condition of the object to be measured can be measured with a higher accuracy.
For example, when unevenness in gloss, the external appearance of a weld, flow marks, the presence of an injury, unevenness in height, or the like of the object to be measured made of a synthetic resin, if the irradiation angle with the light is selected by observing the object to be measured with the naked eye with varying the angle, a highly accurate inspection result corresponding to the eye observation can be obtained on each item.